Residents of two small towns in the Hauraki district have been told to boil their water after heavy rain last week.
Hauraki District Council has hand delivered "boil water" notices to about 100 Mackaytown and 100 Kaimanawa households.
The council says it's "playing it safe" by issuing the precautionary notices.
Samples showed high sediment levels in the Mackaytown water supply while E.coli bacteria has been found in the Kaimanawa water network.
They are part of rural water supply networks that rely on a single-barrier chlorination treatment system.
"At this stage testing for bacteria in Mackaytown has come back clear. However, high levels of sediment, due to increased rainfall, can reduce the effectiveness of chlorine in killing bacteria," says mayor John Tregidga.
The move comes after thousands of people became ill from contaminated water in Havelock North in Hawke's Bay.
Academics have also said intensification of agriculture may cause problems with Canterbury's water supply from aquifers.